Summer’s back and Kiwis are heading to the beach for sun, fun and kaimoana – but algal blooms are back too, and they can bring dangerous biotoxins with them.
Algal blooms are made up of naturally occurring phytoplankton. Just like plants, phytoplankton use sunshine and nutrients (from the water) to grow. If conditions are right, phytoplankton can multiply to high numbers and form algal blooms.
Most species of phytoplankton are harmless, but some can naturally produce highly toxic chemicals called marine biotoxins.
Bivalve shellfish filter out phytoplankton and other particles from the water as a food source. If these algae are toxic, biotoxins can build up in the shellfish, making them poisonous.
“These toxins can cause serious illness or even be fatal, so you should avoid eating shellfish from any area where we have issued a warning,” says New Zealand Food Safety (NZFS) deputy director-general Vincent Arbuckle.
To help prevent people getting ill from eating shellfish containing these biotoxins, NZFS monitors 40 recreational harvesting areas around the country. Water and shellfish are tested weekly or fortnightly for algae or biotoxins respectively. The commercial shellfish industry also provides their own biotoxin monitoring data from more than 30 sites, giving even greater coverage.
If shellfish are found to contain biotoxins at unsafe levels, NZFS issues public health warnings and posts signs at affected beaches.
“To make sure you know what the current biotoxin warnings are, check our website, subscribe to our alerts, or download our free NZ Fishing Rules App.”
Algal blooms die off once the phytoplankton run out of nutrients or conditions change, such as a water temperature dropping or a storm disturbing the water column. Once the bloom disappears, shellfish naturally cleanse themselves of the toxin. This may take a few weeks or months, depending on the type of shellfish.
Shellfish affected by biotoxins usually include bivalves like mussels, oysters, tuatua, pipi, toheroa, cockles and scallops. But pūpū (cat’s eyes) and Cook’s turban can also have elevated levels of some biotoxins. Kina are still safe to eat.
“It’s important to know that cooking will not make toxic shellfish safe to eat, because cooking doesn’t destroy biotoxins,” says Mr Arbuckle.
Generally, you can still eat pāua, crab and crayfish, but only if you remove the gut before cooking, as biotoxins accumulate in the gut. Finfish are not affected by biotoxins from algal blooms, but NZFS advises discarding the liver before cooking.
“If anyone becomes ill after eating shellfish, phone Healthline for advice on 0800 61 11 16, or seek medical attention immediately. We advise that you also contact your nearest public health unit and keep any leftover shellfish for testing.
“Keep yourself and your whānau safe this summer – don’t collect or eat shellfish from areas where we’ve issued a biotoxin warning.”
Check before you collect:
- Subscribe to shellfish biotoxin alerts at https://www.mpi.govt.nz/ news/subscribe-to-mpi/
- See the current warnings here: www.mpi.govt.nz/shellfish
- Download and check the NZ Fishing Rules App www.mpi.govt. nz/rules
- Look out for signage at your local beach.
